Burning time

Burning time is the maximum time to stay unprotected without
receiving sunburn. This is calculated from the UV energy equivalent of 1
MED as the basis.
MED: Minimal Erythemal Dose - Is the effective UV dose that causes a
perceptible minimum reddening/tanning of previously unexposed human skin.
The exact energy equivalent differs by a factor of 2 depending upon
individual sunburn sensitivity.
Each value given in each of the categories represents an approximate
mid-point value for a range. In the monthly solar reports the sensitive
group appropriate to Category 1 is used with a MED of 200 J m-2 effective.